Small GTPases and Vesicle Trafficking: Sec4p and its Interaction with Up- and Downstream Elements

Abstract

Eukaryotic cells are characterized by the presence of many intracellular membrane bounded organelles. Transfer of material from one organelle to another is generally mediated by vesicular transport. By this mechanism, a vesicle buds from a donor organelle and migrates through the cytoplasm until it recognizes the appropriate target organelle (Palade 1975; for a recent review see Rothman and Orci 1992). The fusion of the vesicle membrane with that of the target organelle completes the transfer of both membrane constituents and lumenal contents. The wide variety of potential donors and potential targets necessitates an efficient mechanism for maintaining the specificity of the transport process. This can be separated into two distinct steps. First, the appropriate proteins must be incorporated into a transport vesicle. This process is known as sorting and involves a collection of membrane-bound receptors and cytoplasmically disposed coat proteins that serve to define the vesicle constituents. The second, and equally important step, is the process by which each vesicle is brought into contact with the appropriate target organelle. While cytoskeletal elements are involved in the transport of vesicles to the appropriate region of the cell, a membrane-membrane recognition mechanism also must be involved since different target organelles can exist in very close proximity. Over the last 5 years, much attention has been focused on a particular class of ras-related GTP binding proteins as possible elements of the molecular machinery that generates the high degree of specificity required for this vesicle-target recognition process.